An emerging segment in the bird watching market is for digital camera-based products that automatically take pictures of birds when triggered by motion detection. These cameras use a Pyroelectric Infrared (PIR) sensor to detect motion. The PIR sensor must be very sensitive to detect the typically small heat signature of a bird, and this leads to a problem: The high sensitivity required has a drawback in that it greatly increases the probability of false triggers: Other animals, sun radiation, swinging bird feeders and moving foliage or other objects are examples of events that have the potential to trigger the PIR detector. These false triggers of course lead to pictures without birds in them. In this paper we present a novel approach to detect the motion of birds with a higher rate of success, or percentage of pictures that capture a bird, than the current state of the art. It should be understood that the use of the technology is not limited to photography birds but may also apply to other discrete objects.
A typical PIR motion sensing system includes two main components: the PIR sensor and the Fresnel Lens. The Fresnel lens divides the detection space into multiple sub-detection zones and also creates “dead bands”, areas between subdetection zones due to the discontinuities in the Fresnel lens surface. These dead bands significantly attenuate the IR signature of a body of heat. As a body (or a heat source in the IR frequency range) crosses from one sub-detection zone to the next, the heat signature of the body is sensed sequentially by the PIR sensor. The sensor produces electrical signals proportional to the amount of IR radiation hitting its surface. These signals are monitored and processed by the system electronic circuitry and are interpreted as motion. The strength of the sensor signal depends on the size and direction of motion of the body, the distance of the body from the sensor and the body speed. It should be understood that the strength of the sensed signal will vary as the dead bands are crossed in a repetitive manner, the PIR lens alternating sub-detection zones followed by dead bands will produce alternating relatively strong signals followed by a relatively attenuated signals; all these signals in a contiguous sense comprise one time varying signal that can be characterized by a frequency which is proportional to the speed of the moving body of heat, therefore, the speed at which the dead bands are crossed will give an indication of the speed at which the heat source is moving within the field of view.
Typically manufacturers recommend that a motion detector camera be securely mounted to an immovable object such that the camera is free from movement, inasmuch and the relative motion induced by camera instability can sometimes result in a false reading as the field of view of the motion detector changes. Where a bird feeder is subject to movement due to wind or other environmental influences, the movement of the feeder can in some instances create changes in the thermal energy incident on the PIR and cause a false trigger.